[llvm-exegesis] Allow custom PFM counters

It would be very useful if we could provide our own custom counters to libPFM / perfevent and not just rely on the libPFM event lists.

libPFM usually restricts itself to perf counters / masks in official CPU documentation, which might not be complete, or a counter has been hidden due to minor inconsistencies, even though the counter is still present on hardware and will typically function fine for the cases that llvm-exegesis is interested in.